docs: def _resolve_credential(self, credential):
          """Resolve credential from envars or credentials store.
  
          Args:
              credential (str): Credential to resolve.
  
          Returns:
              str or None: Resolved credential or ``None``.
  
          """
          if self._credentials_found_in_instance:
              return
          elif self._credentials_found_in_envars():
              return os.getenv('PAN_' + credential.upper())
          else:
              return self.storage.fetch_credential(
                  credential=credential, profile=self.profile)